1. Field of the Invention
The present invention relates to device designed for the direct or indirect application of a liquid or viscid coating medium onto a moving material web, in particular, a substrate made of paper or carton. The device includes a coater for applying the coating medium directly or indirectly onto the material web, a device to meter and/or smooth (below referred to as metering/smoothing device), positioned downstream of the of the coater in the feed direction of the material web at a certain distance from the coater, and a support roll around which the material web is guided.
2. Description of the Related Art
A device for application of coating medium onto a web is known from European Patent Document EP 0 770 730 A1. With this device, the material web is guided around a support roll onto which the coater is mounted and serves to pre-meter and apply the liquid or viscid coating medium onto the material web. Additionally, the support roll holds the metering/smoothing device which performs the final metering of the pre-metered coating application, as well as the smoothing of the coating medium, if required. Because of the demands by paper and carton manufacturers to increase the material transfer speed, the state-of-the art coaters have the problem that the freshly-applied coating medium onto the material web is exposed to ever-increasing centrifugal forces. This increase in centrifugal force promotes the formation of projections on the coating layer, whereby the possibility exists that the metering/smoothing device is not capable of entirely smoothing out these projections, compromising the quality of the end product. Since the centrifugal force acting upon the coating medium changes with the square of the material speed and only linearly with the reciprocal value of the roll radius, the problem was partially addressed by increasing the roll radius. Hence, European Patent Document EP 0 690 172 A1 suggested to provide the coater and the metering/smoothing device with an additional support roll in order to allow the material web to traverse in an essentially straight line between the two support rolls. This solution, however, has the disadvantage of occupying a substantial amount of machine space because of the need to use two support rolls.
The present invention provides a generic coater that requires less machine space as compared to the state-of-the art equipment.
A generic device addresses this requirement in accordance to a first aspect of this invention, in which the material web is guided around the support roll only in the area of the metering/smoothing device. The material web traverses in a substantially straight line between the coater and the support roll, and a support arrangement supports the material web in the straight section between the coater and the support roll. This coater occupies relatively little machine space in the direction perpendicular to the feed direction of the material web and, moreover, stabilizes the movement of the material web in the section between the coater and the support roll.
In accordance to a first embodiment, the support arrangement can include a stationary support surface. This stationary support surface can be, for example, a substantially flat support plate on which the material web glides. It is, however, also feasible for the stationary support surface to be a textural support panel, whereby the texture is designed in such a way that an air cushion forms between the material web and the textural surface, on which air cushion the material web moves. Such an air cushion can be formed by air, for example, which is transported by the material web on its outer surface as a result of the boundary layer effect. The latter design has the advantage that the material web can move past the support arrangement substantially frictionlessly, despite the large flat area associated with the support structure.
Finally, it is also feasible to generate an air cushion by expelling air through nozzles which are distributed throughout the support surface, and thus evenly distributing the air across the entire support surface.
With the above mentioned embodiments, it is possible for the support arrangement to reach into the gore formed by the material web and the support roll in order to support the material web as closely as possible to the support roll and far into the metering/smoothing nip. The surface of the support arrangement facing the support roll can thereby be shaped as a flat surface. It is, however, preferred that the surface facing the support roll matches the peripheral shape of the support roll. This allows the surface to reach deep into the metering/smoothing nip. This design further allows a very close proximity between the outer surface of the support arrangement and the surface of the support roll, which has the additional advantage of disrupting the boundary layer formed by the air on the outer surface of the support roll.
A smooth interaction between the support arrangement and the material web can be further assured by providing the feed side of the support arrangement with an approach panel for the material web. The approach panel can be formed by either a rounded off edge, by a sloping panel member positioned at an acute angle relative to the material web, or by a polygon-shaped approach panel. Furthermore, such approach panels have the benefit of easing the feeding process of the material web and further, provide support in the area upstream of the coater.
The support arrangement can be designed as a sheet metal construction, preferably made from stainless steel.
In accordance to an additional embodiment, the support arrangement includes a circulating support belt whose surface facing the material web between the coater and the support roll forms a substantially flat surface. This embodiment also offers a support arrangement for the material web with little or no friction.
As a further development of this embodiment, the support belt is looped around the support roll which guides the material web in the area of the metering/smoothing device. This provides support for the material web all the way to the metering/smoothing device. A particularly stable support can be achieved by providing at least one saddle on the side of the support belt facing away from the material web. In addition, the material web can be, in accordance to this design, supported in the area upstream of the coater through appropriate sizing and placement of the support belt.
In accordance to an additional aspect of this invention, in a generic device, the material web is guided around the support roll only in the area of the metering/smoothing device. The material web traverses substantially in a straight line between the coater and the support roll, and means are provided stabilizing the material web in the straight section between the coater and the support roll. Surprisingly, it has become evident that an adequate stabilization of the material web in the area of the coater can be achieved not only by utilizing the especially provided support arrangement, but also by applying a pre-determined amount of tension to the material web in the area between coater and the metering/smoothing device. This allows the material web to enter past either the top side or the bottom side of a web guide element into the straight section. The material web contacts the web guide roll on its coating side or the side opposing the coating side.
Again, it is advantageous that the device to stabilize the material web provide the stabilization effect in the area ahead of the coater and/or all the way to the support roll.
The web guide roll can be created from an easy-running carrying roll, for example, in order to avoid affecting the tension applied to the material web provided by the device to stabilize or tension the material web. Furthermore, the desired tension can be favorably influenced by the manner in which the web guide roll is positioned.
For example, it is feasible that the web guide roll is designed to extend across the material web and be supported on its ends. It is furthermore advantageous to provide support rolls distributed along the length of the web guide rolls. By using such support rolls, web guide rolls such as light-weight carrying rolls with relatively small diameters can also be applied on material webs that are very wide. Furthermore, it is sensible to power the support rolls in order to reduce the friction between the web guide rolls and the support rolls.
For stabilizing the web guide roll, it can bexe2x80x94in accordance to another embodimentxe2x80x94grouped into a multitude of segments, each of them being supported on both of its ends. Such a segmented web guide roll can be followed by another segmented web guide roll. The segments of the second guide roll are offset in the transverse direction relative to the segments of the previous web guide roll.
In another embodiment, the coater can fulfill the function of the web guide element. In doing so, it is especially advantageous for the coater to be a free-jet type coating device since, with such free-jet coating devices, the material web is turned around by a coating medium cushion, comparable to a so-called xe2x80x9cair-turnxe2x80x9d, in which the material web is turned around on a cushion of air. In some case, it can be advantageous for the coating medium to exit the free-jet coating device not only in direction of the web movement, but also in the opposite direction against the movement of the webxe2x80x94in order to obtain an especially stable coating medium cushion.
It should be further noted that in those cases where the function of web guide roll is carried out by the coater, the freshly applied coating medium is forced towards the material web as a result of the centrifugal forces which helps its adhesion to the material web.
In order to achieve the desired tension of the material web, it is further proposed that an additional web guide rollxe2x80x94preferable a spreader rollxe2x80x94is provided in the area of the coater, on the side of the material web facing away from the coater.
As part of a further development of this invention, it is suggested to make the reaction timexe2x80x94also referred to as xe2x80x9cdwell timexe2x80x9dxe2x80x94which is the amount of time available for the coating medium to bind to the material web in the area between the coating mechanism and the metering/smoothing device, adjustable. To do so, the position of the coater in the feed direction of the material web and/or the position of the metering/smoothing device in the feed direction of the material web around the support roll can be made to be altered. For changing the position of the coater and/or the metering/smoothing device, drive mechanisms of electric, hydraulic, pneumatic or hydro-pneumatic nature or means to be operated in a similar manner can be employed. These devices should be able to facilitate an adjustment even during operation of the coater.
Additionally, the position of the web guide roll can be adjustable in the feed direction of the material web. If, as indicated above, the position of the coater is adjustable, then the position of the web guide roll can be adjusted either independently from the adjustment of the coater or together with the coater. The latter case can be realized by mounting the support arrangement of the web guide rolls onto the coater or to a part, which is solidly attached to it.
The metering/smoothing device can include a doctor blade, a smooth or profiled doctor rod, an air blade or air knife or similar devices.
As already mentioned above, the coater can be of the free-jet type design. It has, independently of the use of such a coater as a web guiding element, the advantage that the jet""s momentum acting upon the material web can additionally be used to maintain the desired tension of the material web without the presence of a support surface.